Semiconductor processing operations are often performed in a semiconductor processing tool, which may feature several semiconductor processing chambers or reactors that are connected to a transfer chamber of some sort; the transfer chamber and the semiconductor processing chambers are typically connected together in a hermetically-sealed manner and kept under vacuum conditions. Wafers are typically provided to, and removed from, the transfer chamber via loadlocks.
The loadlocks are typically connected to an Equipment Front End Module (EFEM), which is usually a structure with a wafer handling robot inside that is configured to move wafers between the load locks and a number of other structures within, or attached to, the EFEM. The internal environment of the EFEM is typically referred to as a “mini-environment”; filtered, dry air is typically constantly flowed through the mini-environment (it is not practical to keep the EFEM at a vacuum, which is why loadlocks are used as a barometric interface between the EFEM and the transfer chamber). Wafers are typically supplied to the EFEM using cassettes of vertically-stacked wafers (e.g., 25 or 30 wafers) called Front-Opening Unified Pods (FOUPs); the robot in the EFEM may pick a wafer from a FOUP and transfer it to the loadlock, or may transfer processed wafers into a FOUP.
Industry standard processes typically mandate that a FOUP contain only unprocessed (and thus clean) wafers or contain only processed (and potentially dirty) wafers—mixing unprocessed and processed wafers in a single FOUP is considered undesirable and is against industry standard practices. Moreover, wafers are often quite hot after they exit processing, and it may be undesirable to put hot wafers into the FOUPs until they are cooled down. To that end, an EFEM will often include a buffer or buffer station that may be used to temporarily store in-process wafers before they are transferred to the processed wafer FOUP. Buffers typically hold a large number, e.g., 25 or 30, wafers in a vertically-stacked arrangement.
Since the wafers that are stored in a buffer have usually undergone recent semiconductor processing operations, they often have chemical residuals from the processing operations on their exposed surfaces. Moisture and oxygen in the mini-environment of the EFEM may react with such chemical residues, causing damage to the processed wafers and can also damage the components of the buffer or EFEM near the wafers.
One technique for mitigating such chemical reactions is to use a buffer such as is described in KR 1020130059574, published Jul. 6, 2013. Such a buffer includes two opposing, vertical stacks of plates that are at least a wafer diameter apart. Small, short ledges extend from alternating instances of such plates and provide a small support area on which each wafer may rest. These ledges extend around only a short distance of the wafer perimeter on opposing sides of the wafer, e.g., around about ⅛th of the wafer circumference on each opposing side. A series of vertical gas delivery tubes extends upwards through each plate instance, and nozzles in each tube are used to direct purge gas out of slots in alternating instances of the plates and towards the mid-plane of the wafers. The purge gas in this case flows towards the middle of the wafers as well as towards the front and the back of the wafer.